Electroplating zinc copper alloys



United States Patent 2,700,646 ELECTROPLATING ZINC COPPER ALLOYS Allan E. Chester, Highland Park, Ill., assignor to Poor & Company, Chicago, 11]., a corporation of Delawarev No Drawing. Application December 7, 1951, Serial No. 260,578

13 Claims. (Cl. 204-44) This invention relates to new and useful electroplating compositions and to a new and improved method for theelectrodeposition of alloys of zinc and copper.

One of the objects of the invention is to provide a new and improved method for extending the range of current densities employed in electrodepositing white alloys of zinc and copper from cyanide-zinc-copper plating baths.

Another object of the invention is to prepare new and useful cyanide-zinc-copper plating baths.

- An additional object of the invention is to provide a new and improved method for electrodepositing bright zinc-copper alloy plates.

Still another object of the'invention is to provide a method of electroplating. alloys of Zinc and copper whereby a bright zinc-copper alloy plate is obtained on polished steel which can be lacquered or chrome plated directly without bufiing. Other objects will appear hereinafter.

In accordance with the invention it has been found thatthe range of current densities at which bright deposits of white alloysof zinc and copper can be obtained is extended by incorporating with the electrolyte from which such alloys are electrodeposited a quantity of dithiobiuret.

Dithiobiuret has the following chemical formula Certain methods are known in the art for electrodepositing white alloys of zinc and copper but in order to obtain a bright plate it has heretofore been necessary to use current densities not exceeding about 20 amperes per square foot. One reason for this appears to be that the amount of copper electrodeposited onto the article to be plated increases as the current density increases. When the relative proportion of copper, as compared with that of zinc in the electrodeposited plate, exceeds a certain amount the desired result is not obtained.

In the practice of the invention to obtain a bright deposit of an alloy of zinc'and copper which is suitable forchromium plating without bufiing it is preferable to employ at least four addition agents in the preparation ofa cyanide zinc-copper electroplating bath, as follows:

l. A purifier, for example, sodium sulfide, sodium polysulfide, ammonium sulfide or ammonium polysulfides.

2. Zinc in the form of a zinc aldonate, preferably zinc gluconate.

3. Dithiobiuret.

4. A brightening agent, preferably the reaction product of an oxyaldehyde and an amine containing primary and/ or secondary amino groups and a water solubilizing radical such as hydroxy, carboxy or sulfonic radicals or salts thereof.

The invention will be illustrated but is not limited by the following example in which the quantities are stated in parts by weight unless otherwise indicated.

Example An electrolyte is prepared by dissolving zinc cyanide, sodium hydroxide and sodium copper cyanide Na2Cu(CN) a in water to produce a bath containing .99 ounce per gallon of'Cu, 4.50 ounces per gallon of Zn, 13 ounces per gal- Ion of total NaCN and 7 ounces per gallon of NaOH. This bath has a pH of about 11.95.

To the above bath there is added 15 to 35 grams per gallon of zinc gluconate.

Approximately 3 cc. per gallon of a sodium polysulfide solution are mixed with the resultant product which is then filtered through a filter cake made up of activated carbon and asbestos in approximately equal proportions.

To the filtrate there is added 0.5 to 12 grams per gallon of dithiobiuret.

To the resultant product there is then added 5 to 30 cc. per gallon of a brightening agent, for example, the product obtained by reacting 10% anisic aldehyde, 5% heliotropin and of mixed isopropanol amines.

Finally the product is filtered through a filter cake made of asbestos or wood pulp only.

The zinc gluconate is prepared by heating a commercial 50% gluconic acid solution to about F. for a time sufiiciently long to melt any crystalline lactone that might be present, then adding one mole of lead-free zinc oxide for every two moles of gluconic acid present and heating the mixture to 170 F. with agitation. The temperature is held at 170 F. for 10 minutes or until solution of the ingredients is complete. The resultant product is cooled to 120 F. in the liquid phase and poured into large stoneware vessels and cooled until a waxy solid forms aldng with some supernatant liquor. The liquor is mainly glucose and water and is removed from the solid by filtration. The residue is placed in dry1ng trays Where it may be washed with ice water to remove traces of residual glucose. The product is then dried by heating at temperatures below 250 F. until it becomes dry and hard, at which time it is ground in a hammer mill or in some other suitable manner.

The'sodium polysulfide solution employed in the foregolng example can be prepared by mixing 1200 pounds of sodium sulfide with 480 pounds of powdered sulfur and making up to 480 gallons with water. I The brightening agent used in the foregoing example is prepared by reacting the anisic aldehyde, heliotropin and the mixedisopropanol amines by heating them together in approximately equimolecular proportions of aldehyde to amine at moderate temperatures up to about F. A slight excess of the amine is preferably employed and the reaction will take place at room temperatures around 75 F. The mixed isopropanol amines conslst essentially of 43% tertiary isopropanol amine, 43% secondary isopropanol propanol amine.

it will be apparent from a consideration of the foregoing example that the zinc is present in the electroplatmg bath as complex ions in the form of sodium zinc cyanide, sodium zincate, and zinc gluconate. The copper is present as sodium copper cyanide NazCu(CN)s.

The zinc gluconate and the dithiobiuret may be considered to be carriers, that is, they function to influence gram refinement in the electrodeposited metal and to extend the range of current densities at which the electrodeposition may be effected without burning the metal.

The anodes employed in electrodepositing an alloy of z nc and copper from a plating bath of the type previously described can be alloys of zinc and copper, for example, an alloy containing 82% zinc and 18% copper. Alternatively, a zinc anode can be used and the copper can be added to the bath chemically. If desired, an external dialysis cell can be used to supply the copper 1ons or both copper and zinc. Because copper dissolves faster than zinc it is also possible to use mixtures of zinc and copper with split anode circuits so that less current is used in dissolving the copper.

Although the bath preferably contains 82 parts of zinc to 18 parts of copper, the range may be varied from about 15 to 30 parts of and still obtain a white alloy of zinc and copper.

During the operation of the electroplating process it is desirable to maintain a ratio between total cyanide and total metal to be electrodeposited within the range of 2.35 to 2.55, preferably about 2.46. This is usually accomplished by adding a sufiicient amount of cyanide to the bath to bring the ratio within the aforesaid range.

amine and 14% primary isocopper to 85 to 70 parts of zinc It will be 'r'ecogniied that there can be some variations and modifications in the manner in which the invention is practiced. The essential feature of the invention is the discovery that dithiobiuret is very effective in making it possible to use higher current densities without burning the electrodepositedplate. For example, although the aldonic acid derivatives, such as zin'c gluco'nate, have an anti-burning effect and make it possible to operate at current densities as high as 25 amper'es per square foot without the addition of the dithiobiuret, with the addition of the dithiobiuret the usable current density is extended to 80 amperes per square foot.

The brightening agent may be omitted from the bath where the article to be plated is to be subjected to a subsequent butfing operation. in general, however, it is preferable to employ a brightening agent in order to obtain a plated article that has sufficient brightness directly out of the bath to permit lacquering 'or the application of a chromium plate Without the necessity of bufiing. Preferred brightening agents for the purpose of the invention are the condensation product of furfural and anthranilic acid, piperonal and anthranilic acid, anisic aldehyde and 2-methyl 'Z-arnino l-propanol, anisic aldehyde and mixed iso'propanol amines, mixtures of piperonal and anisic aldehyde condensed with mixed isopropa'uol amines, mixtures of anisic aldehyde and piperonal condensed with Z-methyl Z-amino l-prop'anol, furfural dispersed with gluconic acid, ani'sic aldehyde dispersed with gluconic acid, piperonal dispersed with gluconic acid, reaction products of gelatin, an aldoriic acid and furfural, reaction products of protein rich seed meals, an aldonic acid and furfural, and other products of a similar nature. The aldehyde condensation products employed as brightening agents are generally characterized by solubility in alkaline aqueous solutions;

When relatively high current densities are used in electroplating white alloys of zinc and copper in accordance with this invention the plating bath described in the foregoing example represents a preferred manner of practicing the invention. However, where it is desired to operate at lower current densities, as in barrel plating, other additives may be employed in the bath. Thus, additional. carriers may be added such as, for example, the protein-aldonicacid reaction products described in U. S. Patent No. 2,458,504. Such protein carriers are reaction products of gelatin or vegetable protein rich seed meals with aldonic acids, i. e., gluconic, mannonic, galactonic and arabonic, or the lactones of these acids.

Although the invention is applicable primarily to the plating of predominantly zinc alloys of zinc and copper which have a silvery or whitish appearance it will be understood that the invention can be applied to the plating of brass generally.

The most important advantage of the invention is the provision of a method and a plating bath which make it possible to plate whitealloys of zinc and copper at much higher current densities than heretofore used. As a result, a plate of a given thickness can be obtained in a much-shorter period of time, thereby speeding up the entire plating operation and greatly increasing the capacity or output of the plating plant.

The employment of the reaction products of oxyaldehydes and amines containing primary and/r secondary amino groups and a water solubilizing group as brightening agents for cyanide zinc-copper electroplating baths is described and claimed in my copending application U. S. Serial No. 246,335, filed September 12 1 951.

The zinc gluconate prepared by the reaction of zinc oxide and gluconic acid is' described and claimed as a new composition of matterin my application U. S. Serial No. 180,500, filed August 19, 1950, which is a division of my application Serial No. 34,127, filed June 19, 1948, which. matured into U. S.- Patent 2,576,997 on December 4, 1951.

The invention is hereby claimed" as follows:

1. A plating bath comprising an alkaline zinc copper cyanide bath containing in solution dithiobiuret, the quantity of dithiobinret being suflicientto extend the usable range of current densities at which white alloys of zinc and copper can be electrodeposite'd from said b'ath.

2. A plating bath comprising an alkaline zinc copper cyanide bath containing in solution a zinc" aldonateand dithiobiuret, the: quantity of dithiobiuretbeing sufiicient to extend the usable range of current densities atwhich "from an alkaline zinc copper cyani can be electrodeposite'd from said bath.

:groups from the gro white alloys of zinc and copper can be electrodeposited from said bath.

3. A plating bath comprising an alkaline zinc copper cyanide bath containing in solution zinc gluconate and dithiobiuret, the quantity of dithiobiuret being suflicient to extend the usable range of current densities at which white alloys of Zinc and copper can be electrodeposited from said bath.

4. A plating bath comprising an alkaline 'zinc copper cyanide bath containing in solution dithiobiuret, the weight ratio of zinc to copper in said bath being within the range of 85 to 70 parts of zinc to 15 to 30 parts of copper, and the quantity of, dithiobiuret being sufiicient to extend the usable range of current densities atwhich white alloys of zinc and copper can be electrodeposited from said bath.

5. A plating bath comprising an alkaline zinc copper cyanide bath containing in solution 0.5 to 12 grams per gallon of dithiobiuret.

6. A plating bath comprisingan alkaline zinc copper cyanide bath containing in solution zinc gluconate, a bath soluble condensation product of an oxyaldehyde, and an amine containing amino groups from the grou consisting of primary and secondary amino groups and a water solubilizing radical from the group consisting" of hydroxy, carboxy and sulfonic radicals arid salts thereof effective as a brightening agent for said bath, and a quantity of dithiobiuret, said quantity being 'siifficier'itto extend the usable range of currentdensities at which white alloys of zinc and copper can be electro'deptisited from said bath.

7. A plating bath consisting essentially of an alkaline aqueous zinc copper cyanide" bath containing 1-5 to 35 grams per gallon of ziiic "luc'onate, 0.5- to 12 grams per gallon of dithiobiuret, 5 to 30 cc. er gallon of the reaction product of by weight anisic aldehyde, 5% by weight heliotrop'in' and 85% by weightmixed isopropanol amines, a weight ratio of total cyanide to total plating metal within therar'ige of 2.35 to 2.55 and a'weight ratio of zinc to copper within the range from 85 to 70 parts of zinc to to parts of copper.

8. A method of electrodepo's'itir'ig an allo'y of zinc and copper which comprises dissolving dithiobiuret in an alka line zinc copper cyanide bath andelectrodepositir'ig an alloy of zinc and copper from said bath onto an object to be plated, the quantity ofdithiobiure't being sufficient to extend the usable range of current densities at which white alloys of Zinc and copper can be electrodeposited from said bath. g

9. A method ofincrea'sing' the usable current density of an alkaline zinc copper cyanide bath which comprise's'dissolving in said bath 0.5 to 12 grams per gallon of dithiobiuret.

10. A method of electrodepositing a white alloy of zinc and copper which comprises electrodep'o'siting said alloy 'de'liath' containing in solution Zinc gluconate and dithiobiuret, the quantity of dithiobiuret being sufiicient toeXtend-the usable range of current densities at which white alloysof zine and copper 11. A method of electrodep'ositirig awhile alloy of Zinc and copper which comprises electrode'positing said alloy from an alkaline zinc copper cyanide bath containing'in solution zinc gluconate,dithiobiuret, and a reaction prod net of an oxyaldehyde and an amine containing amino p consitingpt primary and sec ondary amino groups and a water solubilizing radical from the group consisting of hydroxy, carboxy, anc1; sulfonic radicals andsalts thereof, the;quantity of dithiobiuret being suflicient to extend the usable'range'ofeurrentjdensities at which white alloy sot zincand copper can be electrodeposit'ed from saidbath 12. A method of electrodepositing a white alloy of zinc and copper which comprises electrodepositing said alloy from an alkaline zincledpper cyajide bathcontaining in solution Zinc gluconate, dithiobiuretand 8 5" to 70 parts of zinc to 15 to 30' partsof copper, the quantity of dithiobiuret being sufiicient to extend the usable range of current densities at which white alloys of zinc and copper can be electrodepo'sited from said-bath;

13. A method of electrodepositi'ng awhite alloy of Zinc and copper which comprises electrodepositing said alloy from an alkaline zinc c'opper'-'cyanide bath containing in solution 15 to grams per gallon of zinc gluconate,

, 0.5 to 12grams' per gallon of-dithiobiuret, 5 to-30 cc. per

gallon of the reaction product of 10% bywei'ght anisic 6 aldehyde, 5% by Weight heliotropin, and 85% by weight References Cited in the file of this patent mixed isopropanol amines, a Weight ratio of total cyanide UNITED STATES PATENTS to total plating metal within the range of 2.35 to 2.55 and a weight ratio of zinc to copper Within the range from 85 2,485,5 63 Chester Oct. 25, 1949 to 70 parts of zinc to 15 to 30 parts of copper, at a cur- 5 2,495,629 Chester Jan. 24, 1950 rent density within the range of 20 to about 80 amperes 2,563,360 Phillips et al Aug. 7, 1951 per square foot. 

1. A PLATING BATH COMPRISING AN ALKALINE ZINC COPPER CYANIDE BATH CONTAINING IN SOLUTION DITHIOBIURET, THE QUANTITY OF DITHIOBIURET BEING SUFFICIENT TO EXTEND THE USABLE RANGE OF CURRENT DENSITIES AT WHICH WHITE ALLOYS OF ZINC AND COPPER CAN BE ELECTRODEPOSITED FROM SAID BATH. 